The present invention relates to a non-recrystallized steel sheet which is dip-plated with a Zn-Al alloy and a process for manufacturing the steel sheet. According to the present invention, the dip-plating is carried out at a temperature lower than the recrystallization temperature of the steel sheet without leaving an unplated area so that the plated steel sheet is free of surface defects caused by the presence of an unplated area and possesses a tensile strength of 50 kgf/mm.sup.2 or higher.
Dip-plated steel sheet which uses a molten Zn-Al alloy bath suffers from poor wetting between the plating layer and the steel substrate. Therefore, an alloying reaction of the molten metal with the substrate does not occur on the portions of the surface of the steel substrate where reduction has not been carried out thoroughly during the pretreatment of the steel sheet, usually resulting in surface defects where the substrate is unplated and exposed.
In order to avoid these surface defects, in the prior art, La, Ce, and the like are added to the molten metal bath. However, it is difficult to completely avoid the formation of unplated areas.
In the case of high-strength steel sheet such as ASTM Specification Grade E which possesses a tensile strength of 50 kgf/mm.sup.2 or higher, dip-plating with a molten metal should be performed at a temperature lower than the recrystallization temperature (usually 50.degree. C. or lower) in order to avoid softening of the steel. This processing temperature is rather low in comparison with that for genera-purpose steel sheet which is normally pretreated at 680.degree. C. or higher. Thus, general-purpose steel sheet is annealed for recrystallization during processing.
When high-strength steel sheet is subjected to pretreatment for dip-plating at a temperature lower than its recrystallization temperature, the reduction of the surface of the steel substrate in the heating zone does not occur thoroughly, and the steel sheet just before going into the dip-plating bath has areas of insufficient reduction. This frequently results in the formation of unplated areas, especially in the case of Zn-Al alloy molten metal plating.
Japanese Kokai 59-190332 discloses a method of carrying out dip-plating of a low carbon steel strip using a molten zinc bath after annealing the steel strip at 780.about.820.degree. C. higher than the recrystallization temperature. The low carbon steel contains B together with Ti and Nb so as to improve formability. The tensile strength of the steel is less than 50 kgf/mm.sup.2.
Japanese Kokai 61-60872 discloses a method of carrying out dip-plating of a low carbon steel strip with a Zn-Al alloy. However, in this case, too, recrystallization essentially takes places at an annealing temperature of 780.degree. C. The steel contains 0.025.about.0.12% of Ti together with 0.004% or more of N so as to form TiN which is effective to increase the occlusion of hydrogen of the steel.